Kepler can't directly photograph exoplanets. But the dimming of the parent star they pass in from of (or transit) provides a tell-tale signature of the way the planet is blocking light from the star. When plotting brightness vs. time on a simple graph, a momentary and very small drop in the star's brightness traces a dimple pattern in the stars light curve.

At first glance, it would mean the body was not a spherical planet. The more complicated the pattern the more complicated the silhouette of the body would be. Simulations would eventually be used to reconstruct the geometry.

What if the occulting body turned out to be a square, triangle or diamond in shape? After re-observing numerous times and ruling out every possible natural explanation - and instrument defects - Kepler astronomers would cautiously suggest it was a humongous artificial construction placed into orbit about the star.

A transiting artificial ‘shadowgram' was proposed by Luc F. A. Arnold of the Observatoire de Haute-Provence in Paris in 2005. But now we have an observatory aloft capable of actually testing this hypothesis.

Such data would no doubt be kept confidential until a scientific peer-review. But in the age of the Internet, good luck keeping the wraps on such a bombshell!

But hold on. The extraterrestrial builders would need to make the billboard the size of a terrestrial planet! Therefore the civilization would need to be significantly more advanced than ours, and certainly be a space-faring species with a big budget for super-engineering feats.

Extraordinarily lightweight "gossamer" but opaque material would have to be developed and assemble into a sheet thousands of miles across. It would somehow have to be built to offset the pressure of the stellar wind that would tend to push it away from the star.

The beauty of this idea is that it is a passive signal requiring no broadcast energy or targeting. And, it is detectable over a sizeable volume of the Galaxy. Nor does the billboard require any long-term commitment for maintenance by the host civilization. Set it up and forget it. If the home planet's space budget dries up, so what? In principle a space shade could be designed to last for millennia and even outlast the lifespan of its civilization.

An alternative approach proposed by Arnold is to have a freight train of objects successively transit the star sequentially, and in a distinguishable manner. "At each period, we would observe a series of transits whose number and timing would claim its artificial nature and a will to communicate," he wrote. For example the sequential transit could communicate a series of prime numbers or a binary code.

The billboard would need to be placed close to the star, far inside its habitable zone, so that successive transit could be seen every few days.

Could there be at least once super-civilization within Kepler's field of view that is winking back at us?